NAME
bepos  determine the source position from a rough position list
USAGE
bepos qpoe rough_fname bepos_tab bkden
DESCRIPTION
BEPOS determines the maximum liklihood position for each peak identified by LPEAKS, using the original qpoe file. Signaltonoise ratios in detect cells centered on the final positions are computed and positions with snr's greater than the userdefined threshold are identified as sources.
The algorithm uses the liklihood ratio parameter technique (cf. preprint by w. cash, and memos by s. murray, ssm10278 ans ssm10378). The method was used for the Einstein IPC and ROSAT HRI and given in detail in SAO Special Report 393 by Harnden, Fabricant, Harris, and Schwarz.
previous task to run: LPEAKS, BKDEN
PARAMETERS
 qpoe = "" prompt = Input qpoe file

Name of the input qpoe file. If ROSAT PSPC or Eintein IPC, PI bands must be respecified with the input qpoe filename. (ex. rp110590.qp[pi=52:201], if CELLMAP was run on the hard band)
ROSAT PSPC: soft band = [pi=11:41] hard band = [pi=52:201] broad band = [pi=11:235] Einstein IPC: soft band = [pi=2:4] hard band = [pi=5:10] broad band = [pi=2:10]
 rough_fname = "" prompt = input rough pos table

Required input filename. The file may be specified by supplying the rootname and a "_ruf.tab" will be appended. Each row in the table represents 1 peak position as determined by LPEAKS. BEPOS reads the following table columns of the input file:
x  x pixel position in physical coordinates y  y pixel position in physical coordinates cellx  x detect cell size in arcseconds celly  y detect cell size in arcseconds
 bepos_tab = "" prompt = output bepos table file

Required output filename. The file is written in table format. The file may be specified by supplying the rootname and a "_pos.tab" will be appended. Each row in the table represent 1 source. The column titles and definition appear in the table header and are as follows:
ra  right ascension in degrees dec  declination in degrees x  x pixel position in physical coordinates y  y pixel position in physical coordinates srccnts  source strength (source counts only) in assumed point source. eq: alpha = integral of the PSF over the detect cell beta = integral of the PSF over the detect cell + background cell srcnts = ((16/9)cellcnts  framecnts)/ ((25/9)alpha  beta) bkcnts  background counts in the detect cell. eq: bkcnts = (alpha(cellcnts+framecnts)  Beta(cellcnts)) / ((25/9)Alpha  Beta) cellcnts  counts at final position in a detect cell size box framecnts  bkdensity normalized to a detect cell size box pconf  position error in pixels at 90% confidence snr  signaltonoise ratio C = cellcnts T = cellcnts + bkcnts SNR = ((25/9)*C  T ) / (sqrt(16/9**21)*C + T) cellx  x detect cell size in arcseconds celly  y detect cell size in arcseconds fsflag  false source probability flag: 1 means observed photon distribution is consistent with gaussian PRF for a point source 0 means observed photon distribution is inconsistent with gaussian PRF for a point source
 bkden = "" prompt = Intensity Levels

mean background density in counts/square arcminute. (computed by task BKDEN and written to this parameter as the default value by BKDEN.)
 (prf_sigma = 0.0) [real]

PRF Sigma of the field in arcseconds. When 0.0, the PRF sigma is computed by keying on Telescope, instrument (from input header) & bepos.energy in a PRF lookup table (bepos.prf_table) and using the coefficients that are returned to compute the PRF.
 (energy = 0.0) [real]

Energy (KeV), key look up in PRF coefficients table, coeffs are used to compute the Point Response Function Sigma.
 (prf_table = "xspatialdata$prfcoeffs.tab") [string]

PRF coefficients table. Entries for ROSAT PSPC/HRI & Einstein IPC/HRI. Lookup based on value of bepos.energy. Returns coefficients assiciated with a default lookup when energy = 0.0; returns coefficients closest to the energy when exact match is not found.
 (snr_thresh = 3.0) [real]

Minimum SNR threshold. Sources with computed SNR's below this threshold are rejected.
 (display = 2) [int]

Display info to screen. Integer of range 05. The higher the number more verbose the display.
 (clobber = no) [boolean]

Boolean flag specifying whether or not the output Table file can be overwritten, if it already exists.
 (max_iter = 50) [int]

Maximum iterations for convergence to best position.
 (max_conf_iterations = 20) [int]

Maximum iterations for confidence error calculations.
 (num_conf_lev = 2) [int]

Number of confidence levels that confidence errors are determined for below.
 (conf_= 2.3) [real]

The 68 percent confidence level for minimum cstatistic calculation.
 (conf_= 4.61) [real]

The 90 percent confidence level for minimum cstatistic calculation.
 (refine_limit = 45. ) [real]

Pixel limit between LPEAKS and BEPOS position. When the difference between the 2 positions is greater that this limit the source is thrown out.
 (optimum_radius= 1.8) [real]

Optimum radius sigma. Once the value of sigma has been determined, the maximum likelihood position is determined from events in a circular region of radius ROPT*sigma, where ROPT is this parameter (OPT_RADIUS_SIGMA). This region is calculated once for each input position to this task.
 (c_conv_epsilon = 1.0) [real]

Convergence minimum for cstatistic
 (s_conv_epsilon = 1.0) [real]

Convergence minimum for source
 (cenx_keywd = "CRPIX1" ) [string]

X center header keyword for offaxis angle calculation
 (ceny_keywd = "CRPIX2" ) [string]

Y center header keyword for offaxis angle calculation
EXAMPLES
1.
de> bepos *** PI bands must be Respecified with input QPOE *** Input qpoe file: xdata$rh110267.qp Field background density in cts/arcmin**2 (0.001): 12.8558464 Input rough pos table: harlac24_ruf.tab Output bepos table file: . Input Qpoe: xdata$rh110267.qp bkden = 12.8558464 (cts/sq arcmin), 0.0008928 (cts/sq pixel) 2 Rough Position(s) Input src# ra dec physx physy cellcnts framecnts snr  1 22:08:38.4 45:31:24.2 2380.1 2829.1 95 67 5.3 2 22:08:41.4 45:44:33.4 2323.5 4407.7 1506 394 31.8 Writing 2 Sources to Output file: ./harlac24_pos.tab
BUGS
SEE ALSO
Documentation on running ldetect (a detect task which runs the 5 detect tasks in sequence). (help ldetect )
Documentation on the detect algorithms and philosophy along with limitations in (help explain_detect ).
Documentation giving an overview of the detect package and run order of tasks. (help using_detect )